1. Field of the Invention
The present invention relates to a speaker system, and more particularly to a speaker system in which a plurality of dual-voice-coil speaker units are attached to a bass reflex or Kelton type cabinet.
2. Description of the Related Art
FIG. 7 shows one example of the arrangement of conventional speakers disclosed by W. H. Watkins, "New Loudspeaker with Extended Bass", Audio, Dec. 1974, pp. 38-46. The disclosed speaker intends to improve the sound radiation efficiency in the bass range by utilizing a dual voice coil. In FIG. 7, denoted by reference numeral 100 is a signal input terminal of the speaker, 20 is a dual-voice-coil speaker unit, 21 is a first voice coil, 22 is a second voice coil, 25 is a bobbin to which the voice coils are attached, and 10 is a compensation circuit for keeping constant the electrical impedance of the speaker. The compensation circuit 10 is constituted by a serial circuit made up of an-inductance L and a capacity C.
Generally, when the compensation circuit 10 is not provided, i.e., when only the first voice coil 21 is used, the electrical impedance of the speaker unit 20 forms a parallel resonance circuit and exhibits a characteristic peak near the lowest resonance frequency f.sub.0 of the speaker unit 20, as shown in FIG. 8. Accordingly, a current flowing through the first voice coil 21 is reduced in the vicinity of f.sub.0 and the sound radiation efficiency from the speaker is lowered. With these drawbacks in mind, in FIG. 7, the serial circuit made up of the inductance L and the capacity C is connected in series to the second voice coil 22 to provide such an arrangement that a serial resonance circuit including the resistance of the second voice coil 22 as well is connected in parallel to the first voice coil 21, for thereby always keeping constant the electrical impedance of the speaker and improving the sound radiation efficiency.
FIG. 9 shows variations in sound pressure level in the bass range depending on whether the impedance compensation circuit is present or not. As will be seen from FIG. 9, keeping constant the electrical impedance with the compensation circuit raises the sound pressure level in the vicinity of the lowest resonance frequency f.sub.0, i.e., increases the sound radiation efficiency. That effect amounts to several dB depending on conditions.
FIG. 10 shows a conventional speaker system which is extended, based on the conventional speaker arrangement shown in FIG. 7, to include a plurality of dual-voice-coil speaker units. In FIG. 10, denoted by 10-1, . . . , 10-i, . . . , 10-n are impedance compensation circuits, 20-1, . . . , 20-i, . . . , 20-n are dual-voice-coil speaker units, 21-1, . . . , 21-i, . . . , 21-n are first voice coils, 22-1, . . . , 22-i, . . . , 22-n are second voice coils, and 25-1, . . . , 25-i, 25-n are bobbins to which the voice coils are attached. Note that the same or equivalent parts as those in FIG. 7 are denoted by the same reference numerals.
Meanwhile, to realize the constant electrical impedance in the conventional speaker arrangement, one impedance compensation circuit is required for one dual-voice-coil speaker unit. In FIG. 10, therefore, the impedance compensation circuits 10-1, . . . , 10-i, . . . , 10-n corresponding to the speaker units 20-1, . . . , 20-i, . . . 20-n are required.
In the conventional speaker system thus constructed, when a plurality of speaker units are used, an impedance compensation circuit is required for each of the speaker units. Accordingly, a plurality of impedance compensation circuits must be provided in a speaker system employing the plurality of speaker units. This has raised problems that the-speaker system is generally expensive and a larger space in a cabinet is occupied by the speaker system; hence a volume necessary for reproducing bass cannot be ensured satisfactorily.